Acaricidal and sublethal effects of spinosad on two-spotted spider mites (Acari: Tetranychidae)

Abstract: Toxicity of microbial biopesticide spinosad to life stages of the two-spotted spider mite, Tetranychus urticae Koch, as well as its sublethal effects on this important mite pest were evaluated in laboratory bioassays. The biopesticide was applied to bean primary leaves or leaf discs with spider mites by using a Potter spray tower (2.7 ± 0.2 mg/cm2 aqueous deposit). In acute toxicity bioassays with eggs, larvae, protonymphs and deutonymphs, mortality was assessed based on the number of treated mites reaching the adult stage, while in adult bioassay mortality was assessed 48 h after treatment of preovipositional females. The following LC50 and LC90 estimates for motile stages were obtained (mg/l): 27.52 and 116.72 (larvae), 36.55 and 136.20 (protonymphs), 82.76 and 721.28 (female deutonymphs), and 61.47 and 457.21 (adult females). Spinosad showed no significant ovicidal action: toxic effect observed after spraying eggs (LC50 = 105.78 mg/l, LC90 = 596.95 mg/l) was the result of residual action on larvae that hatched from the treated eggs. Viability and reproduction of adult females that survived treatment with 240, 120 and 60 mg/l were evaluated in two successive 7-day bioassays on untreated leaf discs. In the first bioassay, females were treated as 24 h-old eggs and all treatments significantly reduced net fecundity and the instantaneous rate of increase (ri) of survivors, but the reduction was merely 9-11% and 2-3%, respectively. In the second bioassay, in which females were treated during their preovipositional period, the treatments with 240 and 120 mg/l significantly reduced their net fecundity (28-31%) and ri values (8-9%). Results obtained in this study indicate that spinosad, applied against insect pests (at the recommended rates of 60-240 mg/l), could eliminate a part of T. urticae population as well, but survivors would retain a significant potential for population recovery.Extended abstract